Secretion into blood of pregnant mares
Around day 25 of gestation in mares, the allantois and chorion fuse and form the allantochorion. This leads to formation of the chorionic girdle, which develops at the junction between the enlarging allantochorion and the regressing yolk sac.
The chorionic girdle is a thick band of trophoblast cells and surrounds the conceptus until day 35-38 of gestation. By then, the chorionic girdle cells have proliferated and become mostly binucleate, and they then disassociate from other embryonic membranes and invade the endometrial epithelium.
Upon entering the epithelial stroma the cells stop proliferating and lose their ability to migrate, they enlarge, and terminally differentiate into endometrial cup cells, forming the endometrial cups.
As this happens, between days 38 and 40 of gestation, the endometrial cup cells begin to secrete eCG, and it can be detected in the pregnant mare’s blood.
The endometrial cups reach their maximum activity and eCG its maximal concentration in blood around days 70-80 of gestation, but around that time the cups start degrading until they are completely destroyed.
The amount of eCG secreted varies considerably between mares, both in regard to its concentration and to how long it is secreted in significant quantity.